monitoring: setup: add check for already bound agent
monitoring: setup: add check for already bound agent

The agent_id (hostname in this case) may already be bound, check for this case.

kaustavha/rackspace-monitoring-agent,cp16net/virgo-base,virgo-agent-toolkit/rackspace-monitoring-agent,christopherjwang/rackspace-monitoring-agent,kaustavha/rackspace-monitoring-agent,AlphaStaxLLC/rackspace-monitoring-agent,cp16net/virgo-base,cp16net/virgo-base,cp16net/virgo-base,AlphaStaxLLC/rackspace-monitoring-agent,christopherjwang/rackspace-monitoring-agent,virgo-agent-toolkit/rackspace-monitoring-agent,cp16net/virgo-base